Heretofore, there has been a wafer transferring apparatus including a top ring (wafer holding mechanism) for holding a wafer and a pusher mechanism (wafer transferring mechanism) for transferring a wafer to and from the top ring (see, for example, Japanese laid-open patent publication No. 2001-135604). FIGS. 1A and 1B are views showing a general structure of such a wafer transferring apparatus. FIG. 1A shows the wafer transferring apparatus immediately before a wafer W is released from a top ring 60, and FIG. 1B shows the wafer transferring apparatus when the wafer W is released from the top ring 60 and seated on a wafer rest (wafer tray) 40 of a pusher mechanism 10. The top ring 60 has a plurality of holes 65 defined in a wafer holding surface 60a on a lower end surface thereof. The holes 65 are held in fluid communication with a pressurizing and discharging source (not shown) for supplying and discharging a gas, a liquid, or a mixture thereof. The holes 65 connected to the pressurizing and discharging source provide a wafer attracting and releasing mechanism.
The pusher mechanism 10 includes a shaft 11 supporting thereon a guide stage 15 in combination with a helical spring 16 and a centering mechanism 14. Top ring guides 20 are mounted on an outer periphery of an upper surface of the guide stage 15. The wafer tray 40 disposed within the top ring guides 20 is adapted to be lifted by a push stage 30 supported on the upper end of the shaft 11. When the shaft 11 is lifted by a cylinder (not shown), the top ring guides 20 are brought into abutment against an outer periphery of a lower surface of the top ring 60, as shown in FIGS. 1A and 1B.
When the top ring 60 is supplied with a fluid (a gas, a liquid, or a mixture thereof) from the pressurizing and discharging source in FIG. 1A, the fluid 66 is ejected from the holes 65 in the wafer holding surface 60a, releasing the wafer W from the wafer holding surface 60a and seating the wafer W on the wafer tray 40, as shown in FIG. 1B. A process of releasing the wafer W from the lower end surface of the top ring 60 and seating the wafer W on the wafer tray 40 as the wafer rest of the pusher mechanism 10 will hereinafter be referred to as a wafer receiving process.
For holding the wafer W on the top ring 60, on the other hand, the shaft 11 of the pusher mechanism 10 is lifted to lift the wafer tray 40 from the position shown in FIG. 1B until the wafer W placed on the wafer tray 40 abuts against the wafer holding surface 60a of the top ring 60. The pressurizing and discharging source develops a negative pressure in the holes 65 in the wafer holding surface 60a to attract and hold the wafer W on the wafer holding surface 60a. 
Heretofore, a real time required by the wafer receiving process, i.e., a time required after the wafer is actually released from the top ring 60 until it is seated on the wafer tray 40, varies from wafer to wafer and is not constant. In many cases, wafers are completely released within several seconds after start of process, while it takes 5 to 10 seconds in some cases, and takes 10 seconds or more in other cases. When a number of wafers are processed, it currently takes each wafer a different time to release. The releasing time also differs depending on the wafer processing process and the type of top ring 60, and differs from wafer to wafer even if a plurality of wafers in the same lot are processed under the same conditions.
Heretofore, however, it has been the practice to regard the reception of a wafer as being completed upon elapse of a preset time (expected releasing time) after the wafer receiving process has started, and then lower the pusher mechanism 10 and retract the top ring 60 from a position directly above the pusher mechanism 10, whereupon the wafer transferring apparatus changes to a next process. According to this operating process, however, since the wafer receiving process is not completed until the expected releasing time elapses, a wasteful latency time takes place when the wafer is actually released in a shorter time.
Heretofore, it has also been customary to forcibly end the wafer receiving process when the expected releasing time elapses, and start the next process without confirming whether the wafer is properly released from the top ring 60 or not. Therefore, if the wafer is not properly released in time, an accident such as wafer damage may occur. For preventing such an accident, it is necessary to set an expected releasing time with a sufficient margin, and begin the next process after the expected releasing time of all the wafers has elapsed. Such a practice tends to produce a wasteful latency time.
In recent years, a semiconductor fabricating apparatus, such as a polishing apparatus, needs to meet very strict requirements for increased apparatus throughputs. In particular, it is an important task to shorten the time required to perform a wafer transporting process. Therefore, it is necessary to minimize any wasteful latency time in the wafer receiving process to achieve increased apparatus throughputs. Since a reduction in the cost is also of great importance, accidental damage to expensive wafers needs to be minimized in the wafer receiving process.